The clearance of a cylinder is the amount of space that exists between the face of the piston when it is at the end of its stroke and that of the valve when it covers the port, the piston being at the end of the stroke, and as this space exists at each end of the cylinder, the total clearance for a revolution is twice the above amount.
The clearance at the crank end of the cylinder is reduced by the piston rod passing through it.
The amount of clearance may be measured by the following method, which has been given by Professor John E. Sweet:
See that the piston and valves are made tight, and the valves disconnected; arrange to fill the clearance spaces with water through the indicator holes, or holes drilled for the purpose. Turn the engine on the dead centre; make marks on the cross-head and guide that correspond; weigh a pail of water, and from it fill all the clearance space. Weigh the remaining water, so as to determine how much is used. Then weigh out exactly the same amount of water, turn the engine off the centre, pour in the second charge of water, and turn back until the water comes to the same point that it did in the first case. Make another mark on the cross-head, and the distance between these marks is exactly what you really wish to know; that is, it is just what piston travel equals the clearance. This gives the proportion that the clearance space bears to the space in the cylinder occupied by the steam at the end of the piston stroke. Thus, if it takes one pound of water to fill this space, and to admit the one pound of water the piston must be moved one inch, then the clearance bears the same relation to the capacity of the engine as one inch bears to the stroke of the piston. Thus, under these circumstances, in an engine of ten-inch stroke, it would be said the engine had ten per cent. clearance.
When a cylinder is to be rebored, the boring bar should be set true or central to the circumference of the counterbore, so that the bore of the cylinder may be brought to its original position with reference to the bore of the stuffing box.
Cylinders require lubricating, both to avoid friction and wear of the cylinder bore, as well as of the valve and valve seat. The amount of lubrication required depends upon the degree of tightness of the piston rings, upon the speed of the piston, upon the amount of pressure of the valve to its seat, and upon the method of operating the side valve.
Cylinders with releasing valve gears require freely lubricating, because the closure of the valve depends upon the dash pot, and undue friction retards the closing motion.
The less the movement of the valve at the moment of its release, the easier it is to move it, because the friction is less, and less lubrication is required.
Cylinders are lubricated by automatic oilers placed on the steam pipe of the engine, the oil being distributed over the surfaces by the steam.
Cylinder oilers sometimes have a pump to force the oil in, and in others the steam in the oiler condenses, and the water thus formed floats the oil over the top of a tube, or up to an orifice through which the oil gradually feeds as the condensation proceeds.